2910
Inorg. Chem. 1998, 37, 2910-2919
New Class of Oligonuclear Platinum-Thallium Compounds with a Direct Metal-Metal Bond. 2. Structural Characterization of the Complexes Mikhail Maliarik,†,‡ Katja Berg,† Julius Glaser,*,† Magnus Sandstro1 m,† and Imre To´ th§ Department of Chemistry, Inorganic Chemistry, The Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden, and Department of Inorganic and Analytical Chemistry, Lajos Kossuth University (KLTE), H-4010 Debrecen, Pf. 21, Hungary ReceiVed May 30, 1997
A new series of four binuclear platinum-thallium cyano compounds containing a direct and unsupported by ligands metal-metal bond has been prepared in aqueous solution. The structure of these compounds represented by the formula [(NC)5Pt-Tl(CN)n-1](n-1)- (n ) 1-4 for compound I, II, III, and IV, respectively) was determined by means of multinuclear NMR (195Pt, 205Tl, 13C) supported by Raman spectroscopy. In addition, a trinuclear complex with the formula [(NC)5Pt-Tl-Pt(CN)5]3- is formed in solutions where the Pt/Tl ratio is larger than 1. The compounds exhibit very large one-bond 195Pt-205Tl spin-spin coupling constants, 25-71 kHz; the value for compound I, 71 060 Hz, is the largest reported coupling constant between two different nuclei. The possible reasons for this strong coupling, as well as its variation along the series of the binuclear compounds I-IV, are discussed in terms of the metal-metal bond strength, varying participation of s-electrons in this bond and oxidation state of the metal ions.
Introduction The interest in direct metal-metal linkage between platinum and thallium atoms was initiated by Nagle and Balch, who reported six-coordinated platinum in the structure of trans-Tl2Pt(CN)4 which does not possess the usual for platinum(II) tetracyano salts Pt-Pt linked columnar structure but involves two covalent Pt-Tl bonds.1 The unusual structure and strong luminescence exhibited by this compound initiated several studies of its electronic properties.2-4 Later, a Pt-Tl bond was also found in such compounds as the binuclear [Tl(crown-P2)Pt(CN)2](NO3)5 and in two trinuclear Pt-Tl-Pt species: cis[Tl{(1-MeT)2Pt(NH3)2}2](NO3)‚7H2O6 and (NBu4)2[Tl{Pt(C6F5)4}2].7 The oxidation state of platinum is +2 in all these compounds. Thallium is monovalent in these compounds except the latter one, in which the oxidation state is +2: the presence of an unpaired electron makes the compound paramagnetic. Three compounds containing a TlPt3 cluster unit have also been prepared: [TlPt3(CO)3(PCy3)3][Rh(η-C8H12)Cl2],8 [TlPt6(µCO)6(µ-dppp)3]+,9 and [Pt3{µ3-Tl(acac)}(ReO3)(µ-dppm)3]+.10 The latter compounds contain zerovalent platinum and mono* To whom correspondence should be directed. † KTH. ‡ Permanent address: Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninsky Prospect 31, Moscow 117907, Russia. § KLTE. (1) Nagle, J. K.; Balch, A. L. J. Am. Chem. Soc. 1988, 110, 319-321. (2) Ziegler, T.; Nagle, J. K.; Snijders, J. G.; Baerends, E. J. J. Am. Chem. Soc. 1989, 111, 5631-5635. (3) Weissbart, B.; Balch, A. L.; Tinti, D. S. Inorg. Chem. 1993, 32, 20962103. (4) Dolg, M.; Pyykko¨, P.; Runeberg, N. Inorg. Chem. 1995,. (5) Balch, A.; Rowley, S. P. J. Am. Chem. Soc. 1990, 112, 6139-6140. (6) Renn, O.; Lippert, B. Inorg. Chim. Acta 1993, 208, 219-223. (7) Uson, R.; Fornies, J.; Tomas, M.; Garde, R.; Alonso, P. J. Am. Chem. Soc. 1995, 117, 1837-38. (8) Ezomo, O. J.; Mingos, M. P.; Williams, I. D. J. Chem. Soc., Chem. Commun. 1987, 924-925.
valent thallium. Previous attempts to react PtII and TlIII entities have not resulted in formation of a direct metal-metal bond between these two atoms.11 It can be noted that all metal-metal-bonded compounds mentioned above have been prepared in the solid state and are normally not stable in solution, especially not in water. Recently, we have shortly described the formation and characterization of the first representative of a new class of Pt-Tl cyanide species containing a strong unsupported metal-metal bond, namely, the complex ion [(NC)5Pt-Tl(CN)]- which is formed in aqueous solution by the reaction between PtII and TlIII cyano complexes.12 Here we report a detailed structural study of a whole family of bimetallic cyano compounds obtained in the Pt2+-Tl3+-CN--H2O system in solution and characterized by multinuclear NMR and Raman spectroscopy. Experimental Section Materials. A concentrated (1.45 M) aqueous solution of Tl(ClO4)3 in 3.77 M HClO4 was obtained by anodic oxidation of TlClO4.13 A concentrated acidic solution of Tl(NO3)3 (1.42 M in 2.96 M HNO3) was prepared by direct dissolution of thallium(III) oxide in nitric acid under heating. Solutions of thallium(III) cyanide complexes were synthesized either by the method described previously14 or by a direct dissolution of Tl2O3 in hydrocyanic acid.15 Depending on the synthetic route, the content of TlI in the starting thallium solution varied from 1 to 5% of the total thallium concentration. Potassium tetracyanoplatinate(II) trihydrate (Aldrich, reagent grade) was used without further (9) Hao, L.; Vittal, J. J.; Puddephatt, R. J. Inorg. Chem. 1996, 35, 269270. (10) Hao, L.; Xiao, J.; Vittal, J. J.; Puddephatt, R. J.; Manojlovic-Muir, L.; Muir, K. W.; Torabi, A. A. Inorg. Chem. 1996, 35, 658-666. (11) Nagle, J. K.; Balch, A. L.; Olmstead, M. M. Proc. Chem. Congr. N. Am., 3rd 1988, 545. (12) Berg, K. E.; Glaser, J.; Read, M. C.; To´th, I. J. Am. Chem. Soc. 1995, 117, 7550-7551. (13) Biedermann, G. ArkiV Kemi 1953, 441-455. (14) Blixt, J.; Gyo¨ri, B.; Glaser, J. J. Am. Chem. Soc. 1989, 111, 7784. (15) Maliarik, M. Unpublished data.
S0020-1669(97)00667-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/15/1998
New Class of Oligonuclear Pt-Tl Compounds purification. The solution of sodium tetracyanoplatinate(II) was prepared by precipitation of potassium perchlorate from the aqueous solution of K2Pt(CN)4 with an excess of aqueous sodium perchlorate at 275 K and was used as a stock platinum solution. For some NMR measurements 13C-enriched solutions (99%) were used; in this case TlIII(13CN)n3-n species were prepared using Na13CN salt, whereas Na2Pt(13CN)4 salt was obtained from the exchange reaction between K2PtCl4 and Na13CN in aqueous solution with molar ratio 13CN/Pt ) 6, followed by recrystallization of the tetracyanoplatinate salt. Calcd for Na2Pt(13CN)4‚H2O: Pt, 53.1; Na, 12.5. Found: Pt, 54.4; Na, 12.6. Solutions of Pt-Tl-CN Complexes. Bimetallic Pt-Tl cyano complexes in aqueous solution were prepared by mixing solutions of Pt(CN)42- and Tl(CN)n3-n (n ) 2-4) species at different metal to metal and cyanide-to-metal ratios. The solutions were kept dark to avoid photochemical decomposition.16 13C-enriched bimetallic Pt-Tl cyano compounds were obtained when the starting solutions of Pt(CN)42- and Tl(CN)2+ were 99% enriched using Na13CN. After mixing, the resulting Pt(13CN)42--Tl(13CN)2+ solution with the ratio Pt/Tl ) 1:1 was divided into two portions which were titrated (a) with 1.20 M solution of Na13CN or (b) with 3.32 M HClO4 in order to decrease the concentration of free CN-. This procedure allowed us to cover all soluble binuclear Pt-Tl species, and the recorded 205Tl, 195Pt, and 13C NMR data were sufficient for the determination of their structures. For the study of the trinuclear PtTl-Pt complex the same solutions of 13C-enriched thallium and platinum cyanides were mixed with the ratio Pt/Tl ) 2:1. A technique of partial 13C enrichment was also very helpful for spectral assignment. It was found that the cyano ligands of Pt(CN)42are sufficiently inert with respect to the ligand exchange with the thallium site. This fact allowed preparation of the Pt-Tl compounds by the reaction between Pt(12CN)42- and Tl(13CN)2+ (or between Pt(13CN)42- and Tl(12CN)2+). In the current work this method was used only for the structural assignment of the trinuclear Pt-Tl-Pt complex. Analysis. The analytical methods for determination of the H+, Tl3+, and Tl+ concentrations in the solutions were described previously.14 pH values were measured by a combination electrode connected to a pH meter (Radiometer PHM93). The readings of the electrode were calibrated to pH values using the method of Irving for pH > 1,17 and by direct calibration for pH < 1. NMR Measurements. All NMR spectra were recorded with a Bruker AM400 spectrometer at a probe temperature of 298((0.5) K. Typical NMR parameters for obtaining 205Tl, 195Pt, and 13C spectra were presented in our recent publications.12,14 The chemical shifts were referred (in ppm) toward higher frequency from signals of (a) aqueous solution of TlClO4, extrapolated to infinite dilution, (b) aqueous 0.1 M Na2PtCl6 (4533 ppm to higher frequency from X(195Pt) ) 21.4 MHz at 25 °C), and (c) water-soluble sodium salt of TMS, for 205Tl, 195Pt, and 13C NMR spectra, respectively. Raman Spectroscopy. Raman spectra were excited using premonochromatized 514.5 nm radiation from a Coherent Radiation Laboratories Innova 90-5 argon ion laser at an effective power of approximately 400 mW at the sample. Solution spectra from typically 0.05-0.1 M aqueous solutions in glass cuvettes were recorded by means of Dilor Z24 triple monochromator at a spectral bandwidth of 4 cm-1 using photon counting.
Results A. Characterization of Pt-Tl Cyano Complexes in Aqueous Solution by Multinuclear NMR. (1). Formation of Pt-Tl Species. Reaction between colorless aqueous solutions of Pt(CN)42- and thallium(III) perchlorate or nitrate (with (16) (a) In the previous paper12 we have shown that when the oligonuclear Pt-Tl compounds are exposed to light electron transfer is initiated between the two metal atoms, leading to dissociation of the metalmetal bond. The only thallium product of this redox reaction is Tl+, whereas the oxidation of platinum results in formation of mononuclear cyano complexes of Pt(IV) and a Pt(III) cyano dimer.16b (b) Maliarik, M.; Glaser, J.; To´th, I. Inorg. Chem., submitted for publication. (17) Irving, H. M. L.; Miles, M. G.; Pettit, L. P. Anal. Chim. Acta 1967, 38, 475-478.
Inorganic Chemistry, Vol. 37, No. 12, 1998 2911
Figure 1. 205Tl NMR spectrum of complex I in an aqueous solution containing 50 mM Tl3+ (added as nitrate solution), 50 mM Pt(CN)42(added as potassium solution), and 100 mM KCN at pH 0.34. The asterisk denotes the back-folded signal of complex II.
Pt/Tl molar ratio 1:1) results in an immediate and practically quantitative precipitation of a yellow powder. The only species which could be detected (in small amounts) by 195Pt and 205Tl NMR spectra of the mother liquor after filtration of the precipitate were either Pt(CN)42- (δ ) -214 ppm) or Tlaq3+ (δ ) 2034 ppm), indicating a small excess of either platinum or thallium in the starting solutions. Addition of ionic cyanide (aqueous solution of NaCN) to the systems results in an increase of solubility of the precipitate which dissolves completely at a molar ratio [CN]tot/M g 6 (M ) Pt, or Tl), indicating further coordination of the CN- ligands to the metal ions. Dissolution of the powders is accompanied by appearance in 205Tl NMR spectra of two new groups of triplet signals (with intensity ratio 1:4:1) centered at about 780 (the shift differs slightly from that in nitrate medium) and 1370 ppm and assigned to complexes I and II, respectively (Figure 1 and Table 1). The two small peaks in each triplet arise from the spin-spin coupling between the 205Tl and 195Pt nuclei (195Pt: I ) 1/2, natural abundance 33.8%), and together with the central uncoupled peak they give the intensity ratio of 1:3.9:1 as expected for thallium bonding with one platinum atom in the species. The coupling constants J(205Tl-195Pt) are ∼71 and ∼57 kHz for I and II, respectively. The same values of coupling constants were obtained from 195Pt NMR spectra of these solutions. The latter spectra consist of two pairs of 1:1 doublets, with the ratio of approximately 2.4 between them, due to coupling to the two isotopomers of thallium (205Tl and 203Tl both have I ) 1/2, natural abundance 70.5% and 29.5%, respectively) and are centered at 473 and 383 ppm for I and II (cf. Figure 2), respectively. Along with 205Tl NMR data, this shows that the complexes are heterobinuclear.18 Upon further addition of the cyanide ion to the Pt(CN)42-Tl(CN)2+-H2O system ([CN]tot/M > 6), 205Tl NMR signals (1: 4:1 triplets) of two additional platinum-thallium complexes appear in the spectra. The signals are centered at 1975 (III) and 2224 ppm (IV), and the 195Pt-205Tl coupling constants are ∼47 and ∼38 kHz, respectively. These complexes are also binuclear: in 195Pt NMR spectra they appear as pairs of doublets centered at 184 (III) and 68 (IV) ppm with the same values of coupling constants as those obtained from 205Tl NMR (Table 1). The same result was obtained when Tl(CN)3 (yields compound III) or Tl(CN)4- (yields compound IV) was used as a starting complex for the reaction with tetracyanoplatinate. (18) The complexes I and II are also formed when reaction is carried out between solutions of Pt(CN)42- and Tl(CN)2+ with the molar ratio Pt:Tl ) 1:1.
2912 Inorganic Chemistry, Vol. 37, No. 12, 1998
Maliarik et al.
Table 1. NMR Parameters for Pt-Tl-CN Compounds and Some Related Platinum and Thallium Species in Aqueous Solutiona,b species I d
Tl / PtIV(CN)62I II III IV TlIII(CN)3/g PtII(CN)42-/g V
δTl
δPt
-
0 786 1371 1975 2224 2842 1237
δCA
655 473 383 184 68 -213 599
93.4 101.7 110.6 116.1 117.0
δCB/mc
δCC
1
JTl-Pt
-/0 164.8/1 156.9/2 141.9/3f 147.4/3 -
88.8 90.3 91.1 93.4 97.0 130.2 95.5
2J
Tl-CA
-
1
JTl-CB -
2
JTl-CC -
1
JPt-CA -
71060 57020 47260 38760 -
12746 9743 8446 7270 -
2446 876e 52e 7954
592 452 338 255 -
909 843 783 742 -
25168
4600
-
308
700 180h
2
JPt-CB
1
JPt-CC
200 128 ∼0 -
3
JCA-CB -
806 820 821 832 843 1031 858
30 19 ∼0 -
a All parameters were obtained from the 205Tl, 195Pt, and 13C NMR spectra of the Pt(13CN) 2--Tl(13CN) + systems in aqueous solution. b The 4 2 values of spin-spin coupling constants of 13C and 195Pt to 203Tl are not included. c m, number of cyanides bounded to thallium. d Parameters measured 2I IV 13 e in separate solutions of Tl ClO4 and Pt ( CN)6 . Values calculated using computer program WIN DAISY. f The chemical shift was calculated from the observed exchange averaged signal which results from the exchange between cyanides CB of IV and Tl(CN)4-, taking into account their content in solution. g Parameters measured in separate solutions of TlIII(13CN)3 and Pt(13CN)42-. h 3JPt-CA.
Figure 2. 195Pt NMR spectrum of complex II in an aqueous solution containing 50 mM Tl3+, 50 mM Pt2+, and 300 mM [CN]tot at pH 1.08.
Addition of the ionic cyanide results also in an increase of the basicity of the solution. As a result, for solutions containing IV as the dominating binuclear complex, pH is >8 and the anions Tl(CN)4- and Pt(CN)42- appear in the solution indicating some dissociation of the Pt-Tl species. To study the influence of pH on the bimetallic compounds at constant [CN]tot/M ratio, the Pt(CN)42--Tl(CN)2+ solution was titrated with a 1.0 M solution of sodium hydroxide. Only a small decrease of the concentration of II accompanied by an appearance of III could be detected, while the concentration of I was practically constant in the pH range 0.78-1.58. When pH was further increased, the solution slowly turned brown due to the formation of thallium oxide. Titration of the Pt(CN)42--Tl(CN)2+ solution with acid (3.32 M HClO4) has a stronger effect and results in a decrease of the concentration of species II accompanied by a smooth increase of the relative concentration of I. Thus, at pH ) 0.16 up to 50% thallium in the solution is present in the form of complex I. At the same time, the decrease of pH causes a precipitation of a white powder, [(NC)5PtTl],19 from the solution which restricts the highest available concentration of complex I to ∼25 mM. A similar effect as for the acid titration was obtained when the Pt(CN)42--Tl(CN)2+ solution is titrated by an aqueous solution of AgClO4 (1.83 M). Due to the low solubility of AgCN (2.3 × 10-5 g/100 mL at 20 °C) it readily precipitates from the cyanide-containing solutions, allowing a decrease of the concentration of CN- without significant change of pH; this shifts the equilibrium between the binuclear complexes toward species I.20 (19) (a) In the solid compound [(NC)5PtTl] the platinum-thallium separation is 2.63 Å.19b (b) Eriksson, L.; Glaser, J., Jalilehvand, F.; Maliarik, M., Persson, I.; Persson, P.; Sandstro¨m, M.; To´th, I. To be published.
Figure 3. 205Tl NMR spectra of complex V in aqueous solutions containing 50 mM Tl3+, 100 mM Pt(12CN)42-, and 100 mM Na12CN; the asterisk denotes the low-frequency part of a signal of complex II.
Varying the molar ratio Pt/Tl in the range 1:1-1:4 did not result in the formation of additional Pt-Tl compounds. When an excess of Tl(CN)n3-n (n ) 2-4) was used, the same heteronuclear species were observed as described above, but the amount was limited by the amount of available platinum. A different situation occurred when tetracyano platinum was added in excess. In Pt(CN)42--Tl(CN)2+ solutions with Pt/Tl ratio 2:1, a pentet with intensity ratio 1:7.7:17.3:7.7:1 was observed centered at δTl ) 1237 ppm (Figure 3) (in addition to the triplet signals of complexes I and II). This intensity distribution almost ideally coincides with the values 1:7.8:17.3: 7.8:1 predicted for a 205Tl resonance of a trinuclear Pt-Tl-Pt complex (V) considering all possible combinations of magnetic and nonmagnetic Pt nuclei. The value of the 205Tl-195Pt spinspin coupling constant in this trimer amounts to 25 kHz. 195Pt NMR spectra of this species show two doublets due to coupling of platinum to 205Tl and 203Tl nuclei, centered at 599 ppm (Table 1). No 2JPt-Pt was observed. Titration of the solution with acid led to a decrease of the concentration of the trinuclear species and, as in the case of binuclear species, the equilibrium shifted toward complex I. On the other hand, increasing the concentration of cyanide in the solution resulted in formation of the binuclear complexes II-IV. Further increase of the platinum concentration in the system (up to Pt/Tl ratio of 4:1) did not result in appearance of additional Pt-Tl species. Concluding, altogether five oligonuclear Pt-Tl compounds exhibiting strong coupling between the thallium and platinum nuclei were obtained in aqueous solution. The trinuclear PtTl-Pt complex (V) could be stabilized in solutions with excess platinum. The formation of the four binuclear Pt-Tl complexes I-IV (see Table 1) is governed by the cyanide/metal ratio in (20) Maliarik, M.; Glaser, J.; To´th, I.; W. da Silva, M.; Ze´ka´ny, L. Eur. J. Inorg. Chem. 1998, 565-570.
New Class of Oligonuclear Pt-Tl Compounds
(a)
(c)
Inorganic Chemistry, Vol. 37, No. 12, 1998 2913
(b)
(d)
Figure 4. 205Tl NMR spectra of complexes I-IV, respectively (a-d). Aqueous solution containing 50 mM Tl3+, 50 mM Pt(13CN)42-, and: 100 mM Na13CN at pH 0.25 (a); 150 mM Na13CN at pH 1.04 (b); 200 mM Na13CN at pH 5.52 (c); 300 mM Na13CN at pH 8.66 (d).
Chart 1
the solution. This indicates that the main difference between these species is the number of cyanide ligands, which strongly influences both 195Pt and 205Tl chemical shifts and the spinspin coupling constants, J(195Pt-205Tl), of the complexes. (2). Composition of the Pt-Tl Species and Assignment of the Cyanide Sites. Binuclear Pt-Tl Complexes. The data for complex II were communicated recently.12 Its 205Tl NMR spectrum (Figure 4b) was assigned in terms of an AMX4 spin system for the carbon sites. The 195Pt NMR spectrum of the compound is also in agreement with this assignment. Apart from the coupling to the two thallium isotopes, the platinum signal is split into twenty lines present as doublet of doublets of 1:4:6:4:1 pentets, as expected for the coupling to the three groups of nonequivalent carbon-13 nuclei. The three different cyanide types CA, CB, and CC were assigned on the basis of 13C NMR spectra. The four equivalent cyanides, CC, have both 13C NMR chemical shift and 1J(13CC-195Pt) values typical for platinum cyano complexes (Table 1). It seems likely that the original coordination geometry of the cyanide ligands in PtII(CN)42- is maintained in II and the carbons form a square plane around the platinum atom. The value of the chemical shift for CB is much closer to those for the Tl(CN)n3-n complexes than to those for the platinum cyano species (cf. Table 1), but the information from the carbon-metal coupling constants is less conclusive. Still, these data indicate a direct CB-Tl bonding, whereas the CB-Pt coupling is probably indirect, through two or more bonds. The position of the cyanide CA in the structure of II requires some discussion. Its 13C NMR chemical shift and the coupling
constant 1J(13CA-195Pt) are both in the range characteristic for platinum cyanides, which supports its direct bonding to the platinum atom. At the same time this carbon nucleus shows a very strong coupling to 205Tl (∼10 kHz), typical for thallium cyanides and is more than three times larger than 1J(13CB205Tl) (Table 1). Moreover, CA and CB couple to each other with J ) 30 Hz, whereas no coupling to CC can be detected. Several alternatives can be considered for the location of this cyanide ligand, CA. Four binding modes of cyanide have been reported for binuclear complexes of metal ions. Apart from being coordinated through the carbon at single metal centers (A, Chart 1), different types of cyanide bridging between two metal centers as in B,21,22 C,23 or D24,25 are also possible. The formation of structure B with a CA cyanide forming a linear -CtN- bridge between Pt and Tl atoms is unlikely because of the very strong coupling between 195Pt and 205Tl which indicates that platinum and thallium centers are linked by a direct metal-metal bond. Bridge formation via the CA carbon atom as in C and D could both explain the values of J(13CA-195Pt) and J(13CA-205Tl) and the strong coupling between the platinum and thallium centers because in this case the metal-metal bond (21) Bignozzi, C. A.; Argazzi, R.; Schoonover, J. R.; Gordon, K. C.; Dyer, R.; Scandola, F. Inorg. Chem. 1992, 31, 5260-5267. (22) Scott, M. J.; Lee, S. C.; Holm, R. H. Inorg. Chem. 1994, 33, 4651. (23) Budzichowski, T. A.; Chisholm, M. H. Polyhedron 1994, 13, 20352042. (24) Bartley, S. L.; Bernstein, S. N.; Dunbar, K. R. Inorg. Chim. Acta 1993, 213, 213-231. (25) Curtis, D. M.; Han, K. R.; Butler, W. M. Inorg. Chem. 1980, 19, 20962101.
2914 Inorganic Chemistry, Vol. 37, No. 12, 1998 is strengthened by the presence of the bridging group. However, the equivalence of four CC cyanides observed in the NMR spectra of all the studied nuclei is not consistent with such a structure, unless there is a rapid rotation of the cyanide around the Pt-Tl bond resulting in both a time-averaged 13C NMR signal of CC and a coupling of 205Tl and 195Pt to CC. Such a rotation would be slowed at lower temperature, and at some stage the nonequivalence of the different cyanides of the Pt(CN)4 unit would be observed. However, no temperature dependence was observed for the signal of CC cyanides in 13C NMR spectra of binuclear Pt-Tl compounds in the range 298263 K, i.e. down to the freezing temperature of the solution all four carbon nuclei still give one narrow resonance. Another argument against the presence of bridging cyanide in the structure can be provided by comparison of pairs of coupling constants of JPt-CA and JPt-CC, and JTl-CA and JTl-CB. Due to the fact that bridging atoms have a higher coordination number than terminal ones it is commonly observed that the magnitudes of 1J(M-L) to bridging ligand atoms (L) are smaller than those to corresponding terminal atoms.26 For example, in the trinuclear platinum cluster [Pt3(ButNC)6] the values of 1JC-Pt are 1936 and 357 Hz for terminal and bridging isocyanide ligands, respectively.27 The spin-spin coupling constants of the platinum atom to the CA and to the terminal (CC) carbons have very similar values, whereas for thallium JTl-CA is several times larger than JTl-CB which lessens the probability that the ACN ligand occupies a bridging position between Pt and Tl centers. It can also be noted that the C and D types of cyanide bridging have been found only in homonuclear compounds. Therefore, the only plausible location of the cyanide CA in the complex is a direct coordination to platinum in a trans position to the thallium atom:
Despite some differences in detail, the general appearance of the 195Pt, 205Tl and 13C NMR spectra is common for all the 13C enriched complexes I-IV which indicates similar structures of these compounds. The 205Tl NMR spectra are the most informative (Figure 4): in addition to the large spin-spin coupling between 205Tl and 195Pt, they also show coupling to a 13C nucleus of one cyanide ligand (1:1 doublet) with a large coupling constant (12.7-7.2 kHz, cf. Table 1). By analogy with II, this J(205Tl-13C) can be assumed to arise from the interaction between thallium and the cyanide CA. Values of other coupling constants to the 13C nuclei are significantly smaller (Table 1), and their variation for the compounds III and IV, in combination with the relatively broad signals (200-300 Hz), results in less well-resolved multiplets. Accurate J(205Tl-13C) values for all types of carbon nuclei could be obtained only for complexes I and II, and the number of coupled spins was determined directly from the 205Tl NMR spectra. Below, we present the assignment of the 205Tl and 13C NMR spectra28 for the species I, III, and IV; the latter two cases are supported by the results of spectral simulation. (26) Jameson, C. J. In Multinuclear NMR; Plenum Press: New York, 1987; pp 89-131. (27) Green, M.; Howard, J. A. K.; Murray, M.; Spencer, J. L.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1977, 1509-1514.
Maliarik et al. Complex I. The 205Tl NMR spectrum of I shows (similarly as for complex II) spin-spin coupling to 195Pt, CA, and 1:4:6:4:1 pentets arising from coupling to four equivalent cyanides CC (Figure 4a). This indicates that totally five cyanide ligands are present in the complex and the spin system can be described as MX4 for the carbon sites. The 13C NMR spectrum of this compound is fully compatible with the assignment and exhibits signals of two different groups of cyanide ligands positioned at 93.4 and 90.3 ppm with the intensity ratio 1:4, respectively. Both signals are split into doublets by spin-spin coupling to 205Tl and are further split by 195Pt. The chemical shift value and coupling constants of the more intensive signal are indicative of the CC cyanides, which allows us to assign it to the four carbons of the Pt(CN)4 unit of I. The coupling constant of the remaining carbon to 205Tl has the same value as that available from the 205Tl NMR spectra (see above), confirming its assignment to the CA cyanide, and leads to the following structure of this compound:
Complex III. Careful analysis of the multiplets appearing in the 205Tl NMR spectrum (Figure 4c) showed that they result from spin-spin coupling between 205Tl and two different 13C sites with a similar magnitude of coupling constants, which leads to a deceptively simple spectrum. To establish the spin system and accurate values of the coupling constants, 205Tl NMR spectra were simulated on the base of various models, followed by an iteration. This treatment led to the conclusion that only an A2MX4 spin system (considering only the 13C sites) fits to the experimental spectrum (Supporting Information). Therefore, it can be concluded that III, similarly to II, contains three groups of chemically equivalent cyanides and the only difference is that in III there is one more CB cyanide bound to the thallium:
Fine structure of the 13C NMR spectrum of III becomes observable only when the temperature of the solution is decreased to 273 K. Signals of three types of cyanide ligands can be detected at 156.9, 110.6 and 93.4 ppm with the intensity ratio 2:1:4, respectively, and are coupled to 205Tl and 195Pt (coupling to 203Tl is also observed for the former two signals). These resonances can be unambiguously assigned to the CB, CA, and CC cyanides, respectively (Table 1). In addition, the signal of CB is split into a doublet with J ) 19 Hz. By analogy with II we attribute it to the spin-spin coupling to the CA nucleus (this coupling cannot be observed for the signal of CA due to its relatively large linewidth, 53 Hz). The spin-spin coupling (28) The 195Pt NMR spectra of the 13C-enriched complexes I, III, and IV are fully in agreement with the structures proposed for these compounds on the basis of 205Tl and 13C NMR data. Because 195Pt NMR spectra are less informative and do not provide any additional information, they are not discussed in this paper. A typical 195Pt NMR spectrum is included in Supporting Information.
New Class of Oligonuclear Pt-Tl Compounds constants, J(13C-205Tl), obtained directly from 13C NMR spectra of III (Table 1) are in good agreement with the values calculated from the 205Tl spectra and, together with the chemical shift values and signal intensities, confirm the above structure of the complex. Complex IV. The continuous decrease of the platinumthallium coupling constant and the magnitudes of J(205Tl-13C) for the binuclear complexes I through IV (Table 1), in connection with cyanide ligand exchange, leads to only partially resolved signals with a pseudotriplet structure in the 205Tl NMR spectra of IV (Figure 4d). In contrast to III, several possible spin systems can be found to fit the experimental spectrum; thus, additional information about the number of spins coupled to thallium is necessary. Room temperature 13C NMR spectra of the solutions containing IV as the main species show two relatively narrow peaks (line width ≈ 10 Hz) at 116.1 and 97.0 ppm with the intensity ratio 1:4, and coupled to 205Tl and 195Pt. These peaks were assigned to the CA and CC cyanides, respectively, of the binuclear complex (Table 1). In addition, a broad resonance (line width ≈ 330 Hz) centered at about 143 ppm is detected in the spectra. This signal is due to the fast exchange between the CB cyanides of IV and those of Tl(CN)4- (30% of the total thallium in the solution, δTl(CN)4- ) 145.0 ppm). A decrease of the temperature in order to slow down the exchange process led to an increasing line width of the signal, and at 268 K it is broadened beyond detection; further cooling results in freezing of the sample. Other attempts to slow down the exchange and hence to resolve the signal by diluting the solution or by introducing an inert salt (NaClO4) to decrease the freezing point were not successful. Nevertheless, taking into account the concentrations of IV and Tl(CN)4- in solution (as obtained from 205Tl NMR), the intensity of the signal CB can be calculated, leading to the integral ratio of the three cyanide sites in the compound equal to 1:3:4. Therefore, we propose the following structure of IV:
Simulation, and the following iteration of the 205Tl NMR spectra of IV in terms of A3MX4 spin system (for the carbon sites only) resulted in a calculated spectrum which is in good agreement with the experimental one (Supporting Information) and gave the values of the coupling constants between thallium and carbon: J(205Tl-13CB) ) 52 Hz and J(205Tl-13CC) ) 256 Hz; the latter coincides with the experimental value obtained from 13C NMR (Table 1). Trinuclear Pt-Tl-Pt Complex. The determination of the structure of the trinuclear Pt-Tl-Pt complex in solution was carried out in two steps. First, a selectively 13C-labeled complex was prepared by the reaction between Pt(12CN)42- and Tl(13CN)2+ species. The 205Tl NMR spectrum of the complex showed, in addition to coupling to 195Pt giving a five-line spectrum (natural 13C abundance, Figure 3), splitting of each signal into a 1:2:1 triplet (13C-enriched complex, cf. Supporting Information). This is due to coupling to 13C (cf. Supporting Information) which indicates presence of two equivalent cyanide ligands in the structure. The magnitude of this coupling constant (4600 Hz) is of the same order of magnitude as the values of
Inorganic Chemistry, Vol. 37, No. 12, 1998 2915 Table 2. Characteristic Raman Vibrational Frequencies (cm-1) of Binuclear Pt-Tl and Some Related Cyanide Compounds and Symmetric Stretching Force Constants (N cm-1) for the Metal-Metal Bond complex I II III IV TlIII(CN)2+ d TlIII(CN)3 PtII(CN)42PtIV(CN)62-
ACN
Tl(CN) (BCN)
Pt(CN)4 (CCN)
b 2200 sc 2210 w, sh 2175 m, br 2199 s 2188 m, sh 2210 m 2162 s, br 2191 s 2179 m 2210 w 2161 s, br 2191 s 2179 m 2200 2187 2167 s 2148 m 2210 s 2202 m, sh
Pt-Tl
K (Pt-Tl)a
160 s, br 161 s
1.53
157 s 157 s
1.45
a Only metal atom skeletons were used for calculations.29 b No separate line was observed, probably due to overlap by a very strong band at 2200 cm-1. c The B1 vibration frequency cannot be determined accurately due to overlapping bands from other complexes. d Frequency from ref 34.
JTl-CA of the binuclear Pt-Tl compounds, and we assign it to ACN ligand occupying a trans position to thallium in the structure. When both starting complexes, Pt(CN)42- and Tl(CN)2+, were enriched to 99% in 13C, the 205Tl NMR signals were additionally split into nine lines (cf. Supporting Information) due to coupling to the carbon nuclei of two equivalent Pt(CCN)4 units. These findings are also confirmed by the 13C NMR spectrum of the complex exhibiting (in the same way as for I) signals of only two groups of carbon nuclei with the intensity ratio 1:4 and chemical shifts compatible with assignment to ACN and CCN ligands, respectively. All NMR parameters for the trinuclear complex are given in Table 1. By combining all available data, the following structure of the trimer can be proposed:
B. Characterization of Binuclear Pt-Tl Cyano Complexes in Aqueous Solution by Raman Spectroscopy. Due to specific equilibria between the Pt-Tl species in solution only II and IV can be prepared as dominating bimetallic complexes, even if IV is in equilibrium with Tl(CN)4- and Pt(CN)42-.20 This makes Raman spectra of the solutions containing I, III, and V difficult to interpret. Nevertheless, the Raman data of the bimetallic Pt-Tl complexes give important information, especially concerning the metal-metal bond. In the discussion below we deal mainly with the data on II and IV. A typical feature of Raman spectra of all solutions containing the Pt-Tl compounds is the presence of a strong and sharp band in the low-frequency region, for compounds II and IV at 161 and 157 cm-1, respectively (Table 2), which is characteristic for a metal-metal (M-M) bond stretching.29 The depolarization ratio, r, of these Raman bands, 0.23 and 0.26, respectively, is consistent with an assignment to a M-M′ stretching vibration. Generally, bimetallic compounds with single metal-metal bonds (29) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; John Wiley & Sons: New York, 1986; p 536.
2916 Inorganic Chemistry, Vol. 37, No. 12, 1998 unsupported by bridging ligands display M-M stretching bands at wavenumbers lower than 200 cm-1, while multiple bonding or ligand bridging in conjunction with an M-M single bond leads to higher wavenumbers.30 Our estimated values of the stretching force constants (Table 2) are characteristic for single bonds.30,31 The CtN stretching region of the binuclear species is also informative and can provide additional support for the above structural considerations based on multinuclear NMR data. Spectra of II and IV show both four intense bands of coordinated cyanide. Comparison of these two spectra, together with the information on the mononuclear platinum and thallium cyanide complexes (Table 2), allows us to distinguish vibrations of three different types of cyanide ligands. We assign the broad band at the lowest frequency, 21612175 cm-1, with the intensity increasing from II to IV, and almost the same line width as for the Tl(CN)n3-n species, to the BCN ligands bound to the thallium atom (Table 2). The two sharp bands, between 2179 and 2200 cm-1, in the middle of each set of four bands from the compounds II-IV can be attributed to the symmetric (A1g) and antisymmetric (B1g) stretching vibrations of four equivalent cyanides (CCN) of the Pt(CN)4 unit in D4h symmetry.32 The highest frequency band at 2210 cm-1 for all compounds I-IV is assigned to the ACN ligand (Table 2). Above, when discussing the NMR data, we have considered alternative positions of this cyanide ion in the structures of the binuclear Pt-Tl compounds. The vibration frequency indicates a terminal position of the ACN cyanide in the compounds, rather than a bridging one. It was found for a number of the cyano complexes of metals containing both terminal and bridging cyanides that ν(CtN) vibrational transitions indicative of two types of cyanide ligands are detected separately with the bridging ones appearing at considerably lower frequencies. Thus the binuclear complex anion [Re2(CN)6(dppm)2]2- containing a single Re-Re bond exhibits cyanide bands at 2097 and 2081 cm-1 for the terminal and at 1935 cm-1 for the bridging ligands.24 A similar difference, about 100 cm-1, between the terminal and bridging cyanide vibration frequencies was also observed for the binuclear vanadium compound [V(CO)4(CN)2]24-.33 No stretching vibrations of the cyanide ligand were detected below 2161 cm-1 for the series of binuclear Pt-Tl complexes I-IV, which strongly indicates that no bridging cyanide ligands are present. Moreover, the highest frequency, 2210 cm-1, of the cyanide ligands in these complexes is consistent with strong s donation from the carbon centered HOMO resulting in strengthening of the CtN bond.23 Such a structural arrangement is also consistent with the constant frequency of the ACN cyanide stretching vibration in the series of compounds I-IV, which indicates that the Pt-ACN bond strength is almost not affected by the change of the number of cyanide ligands bound to thallium, and therefore the influence of cyanides (BCN)n on the ACN group is not important. Discussion Structure of the Pt-Tl Species. Summarizing the results described in the previous section, four closely related binuclear (30) Shriver, D. F.; Cooper, C. B. In AdVances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1980; Vol. 6, pp 127-157. (31) Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal Atoms, 2nd ed.; Clarendon Press: Oxford, 1993; p 787 and references therein. (32) Memering, M. N.; Jones, L. H.; Bailar, J. C. Inorg. Chem. 1973, 12, 2793. (33) Render, D. J. Organomet. Chem. 1972, 37, 303-312.
Maliarik et al. compounds with a general formula [(CN)5Pt-Tl(CN)n-1](n-1)(n ) 1-4), and a trinuclear complex [(NC)5Pt-Tl-Pt(CN)5]3can be obtained in aqueous solution. The coordination environment of the platinum atom in the bimetallic Pt-Tl species consists of four equivalent cyanide ions forming a square plane around the metal, while the apical positions of the pseudooctahedron are occupied by one cyanide ligand and the thallium atom. Information available from NMR measurements is not sufficient to clarify the structural arrangement of the cyanide ligands bound to thallium in the family of binuclear compounds and only general considerations based on comparison with related species could be presented. Recently, thallium(III) cyanide complexes in aqueous solution were thoroughly studied by using a combination of experimental methods, including X-ray techniques (LAXS, EXAFS).34 It was shown that the Tl(CN)2+ complex has a linear structure with four weakly coordinated water molecules in equatorial positions. It seems reasonable that the linearity is preserved also when a cyanide ligand is replaced by a Pt(CN)5 group, as in II. In contrast to complex II, this linear geometry is not present in III and IV as can be inferred from the observed equivalence of two and three BCN ligands, respectively. When the Tl(CN) complex in 3 aqueous solution is formed from Tl(CN)2+, the coordination number of thallium changes from 6 to 4; a pseudotetrahedral structure for Tl(CN)3(H2O) was assumed.34 A similar pyramidal arrangement of cyanide ligands around thallium may be present in the structure of [(NC)5Pt-Tl(CN)3]3- with the platinum atom of the Pt(CN)5 unit completing the pseudotetrahedral structure. For the other [(CN)5Pt-Tl(CN)n-1](n-1)- complexes in solution (n ) 1-3), the remaining coordination sites of the thallium atom are probably occupied by water molecules. The 205Tl NMR chemical shift of the [(NC)5Pt-Tl-Pt(CN)5]3- complex is very close to that of II. The magnitudes of both JTl-Pt and JTl-CA are only about half as large as those for II. This can be interpreted as a result of substitution of BCN ligand in II by a Pt(CN) unit to form a linear NC-Pt5 Tl-Pt-CN axis. In the structure of the Pt-Tl-Pt trimer the thallium atom can be considered as bridging between the two platinum moieties. Very recently the complex anion [Tl{Pt(C6F5)4}2]2- containing a linear Pt-Tl-Pt core with all metal atoms in oxidation state +2 has been prepared and structurally characterized.7 The linear trinuclear M-Tl-M fragment was also found to be present in the complex anion [Tl{Cr(CO)5}2]which contains, according to the authors,35 Tl1- as a fourelectron building block. On the other hand, when thallium is in the oxidation state +1, the M-Tl-M structure is strongly bent due to a stereoactive electron lone pair at the central TlI; this was observed for complexes with M ) Pt 6 and Ir.36 Formation of a diamagnetic compound [(NC)5Co-Tl-Co(CN)5]5- (closely related to V) obtained from the reaction of thallium(I) with pentacyanocobaltate(II) ions in aqueous solution is also discussed in ref 37. For platinum, this type of Pt-M-Pt coordination geometry seems to be relatively common. Trinuclear complexes with linear Pt-M-Pt entities have previously been reported for M (34) Blixt, J.; Glaser, J.; Mink, J.; Persson, I.; Persson, P.; Sandstro¨m, M. J. Am. Chem. Soc. 1995, 117, 5089-5104. (35) Schiemenz, B.; Huttner, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 1772-1774. (36) Balch, A. L.; Nagle, J. K.; Olmstead, M. M.; Reedy, P. E. J. Am. Chem. Soc. 1987, 109, 4123-4124. (37) Crouch, E. C. C.; Pratt, J. M. J. Chem. Soc., Chem. Commun 1969, 1243.
New Class of Oligonuclear Pt-Tl Compounds ) Pb(II),38 Cu(II),39 and Ag(I).40 In all these compounds the platinum atom has the oxidation state +2 and the coordination number five in a square-pyramidal geometry, in contrast to the six-coordination found in the Pt-Tl-Pt complex prepared in this work. Metal-Metal Bonding in the Pt-Tl Species. The most striking feature of the studied bimetallic complexes is certainly the direct Pt-Tl metal-metal bond which is convincingly detected by both the spin-spin coupling constant between 195Pt and 205Tl nuclei and the stretching frequency ν(Pt-Tl). There are seven known compounds containing a direct PtTl linkage.1,5-10 Vibrational data for the metal-metal bond in these complexes are lacking, while values of 1J(195Pt-205Tl) were obtained for two of them, namely [Tl(crown-P2)Pt(CN)2]+ (3825 Hz)5 and [TlPt3(CO)3(PCy3)3]+ (2032 Hz)8 both with monovalent thallium. As can be seen from Table 1, the platinum-thallium coupling constants of the oligonuclear complexes I-V surpass the previously known values by an order of magnitude. When spin-spin coupling constants involving such heavy elements as platinum and thallium are considered, relativistic effects have to be taken into account.26 This is especially important for thallium due to the presence of s valence orbitals. The relativistic contraction of the 6s shell of thallium leads to an increase of 6s electron density on the nucleus. This in turn contributes to the dominant Fermi contact term (defined as being due to 6s orbitals) for spin-spin coupling41 and is reflected in the large values of J(205Tl-X) even for coupling with lighter atoms (X ) 1H, 13C, and 31P42,43). Thus, for instance, 1J(205Tl-13C) is 14 636 Hz in the TlIII(CN)2+ cation.14 In this light, the dramatic difference in the 1J(195Pt-205Tl) values between the series of species I-V and the other Pt-Tl compounds may reflect varying proportions of thallium 6s character in the metal-metal bond caused by the oxidation state of the thallium atom. This phenomenon can be qualitatively considered in terms of the ns2 “inert pair” effect which is a classical feature some p-block elements of the periodic table.44,45 When in the monovalent oxidation state, the thallium atom essentially preserves its 6s2 filled shell. The minimal participation of thallium(I) s electrons in bond formation is supposedly reflected by the very limited TlI-X spin-spin coupling. Indeed, although numerous compounds of TlI have been investigated using the 205Tl NMR technique, only a few spin-spin coupling constants have been reported42 and even the largest of them are substantially smaller than those for TlIII. An analogous, but not so strong lone pair effect has been observed for several complexes of main group elements and is usually associated with a negative contribution to the one-bond coupling constant. The removal of the lone pair results in large amplification of the coupling.26 In contrast to thallium which exhibits only a few examples (38) Uson, R.; Fornies, J.; Falvello, L. R.; Uson, M. A.; Uson, I. Inorg. Chem. 1992, 31, 3697-3698. (39) Schreiber, A.; Krizanovic, O.; Fusch, E. C.; Lippert, B.; Lianza, F.; Albinati, A.; Hill, S.; Goodgame, D. M. L.; Stratemeier, H.; Hitchman, M. Inorg. Chem. 1994, 33, 6101-6110. (40) Uson, R.; Fornies, J. Inorg. Chim. Acta 1992, 198-200, 165-177. (41) Pyykko¨, P.; L., W. Mol. Phys. 1981, 43, 557-580. (42) Hinton, J. F. M.; K. R.; Briggs, R. W. Prog. NMR Spectrosc. 1988, 20, 423-513. (43) Balch, A. L.; Neve, F.; Olmstead, M. M. J. Am. Chem. Soc. 1991, 113, 2995-3001. (44) Downs, A. J. In Chemistry of Aluminium, Gallium, Indium and Thallium; Downs, A. J., Ed.; Blackie Academic & Professional: London, 1993; p 1-80. (45) Glaser, J. In AdVances in Inorganic Chemistry; Sykes, A. G., Ed.; Academic Press: San Diego, 1995; Vol. 43, p 1-69.
Inorganic Chemistry, Vol. 37, No. 12, 1998 2917 of spin-spin coupling to another metal nuclei, numerous couplings between platinum and different metal nuclei can be found in the literature. A collection of the values of 1J(195PtM) displays a remarkably wide range with little relationship to the metal oxidation state and the structure of the complex.46 The magnitude of the 195Pt-205Tl coupling constant found for the bimetalic complex I is not only the largest known between these nuclei, but also the largest hitherto reported between two different nuclei. The only larger coupling constant is that found for the homopolyatomic cation Hg32+ with a covalent Hg-Hg bond.47 Similarly as for the platinumthallium species, the enormous isotropic coupling constant, 1J(199Hg-199Hg) ) 139 600 Hz, is due to the high 6s contribution to the Hg-Hg bond and the high 6s electron density at the mercury nucleus caused by the relativistic effects on this heavy atom. Therefore, the large values of J(195Pt-205Tl) observed for the compounds I-V allow to exclude presence of monovalent thallium in the series of binuclear species. At the same time, in contrast to the recently prepared paramagnetic complex [Tl{Pt(C6F5)4}2]2+ containing TlII,7 the narrow NMR signals of the bimetallic species obtained in this work exclude the presence of unpaired electrons in the Pt-Tl complexes in solution. Despite the difference in magnetic properties of complexes, the donor-acceptor model proposed by Uson for the Pt-Tl bonding situation7 seems to be appropriate also in the present case. Thus, for the complexes I-V the metal-metal interactions can be qualitatively interpreted as a donation of a pair of electrons from the PtII atom to the s valence orbital of TlIII. An analogous dative bond formalism was used earlier for an isoelectronic PtII(d8) f HgII(d10) system.48 Previous theoretical studies of PtII(CN)42- anion, which is closely related to the bimetallic Pt-Tl species I-V (all containing Pt(CN)4 units), showed 91% contribution of atomic platinum orbitals (mainly 5dz2) to the highest occupied molecular orbital.2 The formation of a σ bond with contributions from the 6s orbital of thallium and the filled 5dz2 orbitals in planar d8 complexes1 is symmetry-allowed and gives rise to a strong covalent Pt-Tl bond in the family of bimetalic complexes. We have already noted that a strong dependence of all NMR parameters on the number of cyanide ligands bound to the thallium atom is a characteristic feature of the Pt-Tl compounds I-IV. Table 1 shows a clear tendency of decreasing 1J(195Pt-205Tl) with increasing number of CN- in the binuclear species. Thus, when going from I to IV, an almost 50% decrease of the Pt-Tl coupling constant takes place. Taking into account the general trend of spin-spin coupling constants to decrease with increasing interatomic distance between the two bound atoms, it would be natural to attribute such a strong effect to a significant weakening of the bond between the two metal atoms. Nevertheless, when dealing with metal-metal bonds, especially between heavy atoms, the situation is not so clear. The lack of correlation between 1JPt-Pt and such indicators of Pt-Pt bond strength as ν(Pt-Pt) and interatomic distance has been discussed for several compounds.49,50 Also, no relation was observed between the metal-metal separations and the (46) Goodfellow, R. G. In Multinuclear NMR; Plenum Press: New York, 1987; p 521-561. (47) Gillespie, R. J.; Granger, P.; Morgan, K. R.; Schrobilgen, G. J. Inorg. Chem. 1984, 23, 887-891. (48) Krumm, M.; Zangrando, E.; Randaccio, L.; Menzer, S.; Danzmann, A.; Holthenrich, D.; Lippert, B. Inorg. Chem. 1993, 32, 2183-2189. (49) Pregosin, P. S. Coord. Chem. ReV. 1982, 44, 247-291. (50) Appleton, T. G.; Hall, J. R.; Neale, D. W.; Ralph, S. F. Inorg. Chim. Acta 1983, 77, L149-L151.
2918 Inorganic Chemistry, Vol. 37, No. 12, 1998 corresponding spin-spin coupling constants in trichlorostannate complexes of platinum(II) containing a direct Pt-Sn bond.51 This observation was referred to a very high sensitivity of the spin-spin coupling to electronic changes in the metal-metal bonding which does not significantly affect other properties of the bond.52 Hence, taking into account the governing role of the 6s orbitals of thallium in the Pt-Tl coupling, marked changes in the values of 1JPt-Tl in complexes I-IV may arise from variation of the s character in metal-metal bonding resulting from coordination of extra CN- ligands to the thallium atom. To assess the Pt-Tl bonding situation in the family of binuclear species Raman stretching vibration of the metal-metal bond was helpful. In contrast to the values of 1J(195Pt-205Tl) obtained from NMR, which decrease by 35% when going from II to IV, the vibrational transition frequency of the Pt-Tl bond changes by only 4 cm-1 (Table 2). The corresponding decrease of the force constants is quite small (5%), which means that the metal-metal bond strength is only very slightly affected by the number of cyanides bound to the thallium atom. Taking into account both the large masses of the coupled metal atoms and the known weakening of the Tl-C bond in Tl(CN)n3-n species upon increasing n,34 it is reasonable that the overall strength of the Pt-Tl bond is only slightly sensitive to coordination of additional cyanide ligands to the thallium atom of the bimetallic compounds.19 Oxidation State of the Metals in the Binuclear Pt-Tl Complexes. The chemical shifts of 205Tl NMR are usually very indicative of the oxidation state of thallium. Thus, in aqueous solution the range of chemical shifts for TlIII is usually between +1800 and +3600 ppm (excluding a few heavy halide compounds) and for TlI between -200 and +200 ppm.42,45 205Tl NMR chemical shifts for the species I-IV lie in the wide range between the values characteristic for TlI and cyanide complexes of TlIII (Table 1 and ref 14). The lowest δTl found for I is 786 ppm which is much closer to the range of monovalent than to trivalent thallium. Coordination of additional CN- ligands to the thallium atom results in the same trend as for the Tl(CN)n3-n complexes, namely an increase of the chemical shift (Table 1); for complex IV the shift is close to the range of TlIII(CN)n3-n species (2300-3000 ppm). Similarly as for thallium, the values of 195Pt NMR chemical shifts for the I-IV complexes lie between the two oxidation states of platinum in its cyano complexes, PtIV (Pt(CN)62-) and PtII (Pt(CN)42-), but with the opposite trend of the δPt change with increasing n in [(NC)5Pt-Tl(CN)n-1](n-1)- species (Table 1). δPt is largest for I, only 180 ppm from the corresponding value for hexacyanoplatinate(IV), whereas δPt for IV is only 280 ppm from tetracyanoplatinate(II). Therefore, in the studied binuclear complexes the decreasing shielding of the thallium nucleus when coordinating additional cyanide ligands is accompanied by an increasing shielding of the platinum nucleus. This is in good agreement with the donor-acceptor model of the Pt-Tl bond considered above. In the absence of a direct Tl-CN bond (as in I), the electron pair donated by platinum is largely localized on the valence orbitals of thallium and, accordingly, gives the highest s contribution to the Pt-Tl bond (for the series of the binuclear compounds) and the strongest coupling constant 1J(195Pt-205Tl). Coordination of the cyanide ions to thallium results in a smooth transfer of electron density (51) Albinati, A.; Naegeli, R.; Ostoja Starzewski, K. H. A.; Pregosin, P. S.; Ruegger, H. Inorg. Chim. Acta 1983, 76, L231-L232. (52) Appleton, T. G.; Hall, J. R.; Neale, D. W. Inorg. Chim. Acta 1985, 104, 19-31.
Maliarik et al. from thallium back to platinum, decreasing the s-character of the metal-metal bond, which is in turn reflected by a decrease in the Pt-Tl spin-spin coupling and the opposite trends of the δPt and δTl values. Discrete oxidation states cannot be assigned to the metal atoms in these complexes, which can be considered to contain partially oxidized (between 2+ and 4+) platinum53 and partially reduced (between 3+ and 1+) thallium. A number of compounds comprising intermediate oxidation states of the metal in the range +2.25 to +2.40 have been found for platinum.54 Complexes of platinum(III) are less common compared to PtII and PtIV but have been intensively studied during the past decade.55 The vast majority of PtIII compounds are binuclear and contain a Pt-Pt entity.31 No partially reduced compounds have been reported for thallium. The only intermediate which can be considered between the two main oxidation states of thallium is the thermodynamically unstable TlII. Until now only three solid compounds of thallium(II) have been reported. Apart from the paramagnetic cation [Tl{Pt(C6F5)4}2]2+ discussed above,7 two other compounds, both containing a direct TlII-TlII bond, have been prepared: the diamagnetic [(Me3Si)3Si]2Tl-Tl[Si(SiMe3)3]256 and the cluster Tl0.8Sn0.6Mo7O1157 with unknown magnetic properties. Conclusions Reaction between Pt(CN)42- and Tl(CN)n3-n (n ) 1-4) complexes in aqueous solution results in the formation of a family of heteronuclear cyano complexes. Depending on the molar ratio Pt/Tl, cyanide concentration, and pH, four binuclear complexes I-IV with the general composition [(CN)5Pt-Tl(CN)n-1](n-1)- (n ) 1-4) and a trinuclear species [(NC)5PtTl-Pt(CN)5]3- (V) can be obtained. Multinuclear NMR, supported by Raman spectroscopy, allows us to establish the stability of the metal-metal bond in aqueous solution and to distinguish three types of cyanide ligands in the complexes. Four equivalent cyanide ions form a square plane around the platinum atom, and one cyanide occupies an apical position trans to the thallium atom in the pseudo-octahedrally coordinated platinum. The number of cyanides bound to thallium varies from 0 to 3 in the binuclear species giving rise to four different complexes. Data on nuclear spin-spin coupling constants and vibration frequencies are fully indicative of a direct and unsupported PtTl bond in the oligonuclear complexes formed in solution. The estimated force constants for the Pt-Tl bonds are characteristic for single bonds. Taking into account the diamagnetic nature of the complexes in solution, the bonding situation in the PtTl systems can be interpreted as a donation of two electrons which are shared by the two metals, where the thallium-related molecular metal-metal orbital accommodates electron density donated by platinum. Quantum mechanical calculations are clearly needed to clarify the details of the bonding situation between the two metals. The strength of the metal-metal bonds as represented by the force constant is only slightly affected by change of the number of cyanide ligands in the binuclear complexes. On the other (53) This is also supported by the cyanide stretching vibrations of the Pt(CN)4 unit in the family of the binuclear Pt-Tl compounds exhibiting ν(CtN) values in the range between the frequencies of PtII and PtIV cyano complexes (Table 2). (54) Williams, J. M.; Johnson, P. L.; Schultz, A. J.; Coffey, C. C. Inorg. Chem. 1978, 17, 834-839. (55) Roundhill, M. D. In ComprehensiVe Coordination Chemistry; Pergamon Press: Oxford, 1987; Vol. 5, p 375-500. (56) Henkel, S. Klinkhammer, K. W.; Schwarz, W. Angew. Chem., Int. Ed. Engl. 1994, 33, 681-683. (57) Dronskowski, R.; Simon, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 758-759.
New Class of Oligonuclear Pt-Tl Compounds hand, the value of 1J(195Pt-205Tl) changes significantly with n in [(CN)5Pt-Tl(CN)n-1](n-1)-. Assuming that contact contribution dominates the spin-spin coupling mechanism, the large difference in the Pt-Tl coupling constants, varying between 71 and 38 kHz, may arise from different amount of thallium 6s character in the bond. This can be viewed as a shift of the electron density between the two metals due to the influence of additional cyanide ligands bonded to the thallium atom. Such an electron transfer is consistent with a smooth change of (i) shielding of the 195Pt and 205Tl nuclei, reflected by the values of their chemical shifts, which fall in the region between those for PtII(CN)42- and PtIV(CN)62-, and TlIII(CN)n3-n and TlI, respectively, and (ii) CtN stretching frequencies lying in the range between bi- and tetravalent platinum cyanides. This shows that we deal with partially oxidized/reduced metal atoms. The most pronounced transfer of the electron pair between the two coupled metal centers takes place in the absence of cyanide ions coordinated to thallium; hence, the cyanide ligand stabilizes trivalent thallium and prevents its reduction by platinum(II). Intermediate oxidation states are well-known for platinum, but for thallium this is the first report of formation in solution of a series of stable compounds containing diamagnetic thallium with oxidation state between +3 and +1. The dependence of the oxidation state of the metals on the number of the cyanide ligands coordinated to thallium in heterobinuclear complexes
Inorganic Chemistry, Vol. 37, No. 12, 1998 2919 can be used for controlling the redox process between the two bound metals and the electronic properties of the intermediates formed can be studied with particular attention to their sensitivity to photoinduced electron-transfer reactions. Acknowledgment. The continuous financial support of the Swedish Natural Science Research Council (NFR) is gratefully acknowledged. The authors thank Carl Trygger Foundation for Scientific Research, Wennergren Center Foundation, the Hungarian National Scientific Research Foundation (OTKA T017152), High Education Research and Development Grant (0432/1997), and the European Commission INTAS Program for financial support. M.M. acknowledges the Swedish Institute for a postdoctoral fellowship. Supporting Information Available: Output parameters from simulation of 205Tl NMR spectra (spin-spin coupling constants, chemical shifts, line widths), figures showing experimental and simulated 205Tl NMR spectra of aqueous solutions of III and IV, 205Tl NMR spectra of partially and fully 13CN-enriched complex V, Raman spectra (region of stretching vibrations of the CN group) of aqueous solutions of II and IV, and diagram showing 205Tl and 195Pt NMR chemical shifts of I-IV as a function of the number of cyano ligands bound to thallium (8 pages). Ordering information is given on any current masthead page. IC970667V
